The present invention relates to a semiconductor laser apparatus with an improved feedback control circuit for controlling the driving of the semiconductor laser, an information recording/reproduction apparatus such as an optical disk apparatus, and an image recording apparatus such as a laser beam printer or a copying machine.
Semiconductor lasers have advantages such as an easy direct light intensity modulation, small in size, a low consumption power and a high efficiency, and therefore they are widely used in memory devices having large capacities, such as optical disk devices and laser beam printers.
However, the semiconductor laser of the current type entails drawbacks, that is, the amount of light emitted is varied, and the laser itself is easily damaged due to excessive current, for various reasons.
When a semiconductor laser is driven, the amount of light emitted from the semiconductor laser is detected, and the driven amount is negative-feedback-controlled on the basis of the result thereof, and a control circuit for stabilizing the amount of light emitted is used. Further, in the case of the optical disk device, there are great demands of a light intensity modulation of a higher accuracy and lower noise during reproduction of data, in order to achieve a larger capacity and a higher speed of data transfer.
In particular, in a recordable medium or a phase changeable medium (PC medium) capable of overwriting data, the S/N ratio of the entire system greatly depends upon optical laser noise, and therefore it is very important to suppress such noise in order to increase the recording density.
Further, in a magneto-optical medium (MO medium), the differential detection method is used for the reproduction of a signal, and therefore the influence of laser noise is less as compared to the case of the recordable medium or PC medium. However, since the reproduction signal level is extremely low, it is required that the optical laser noise should be suppressed to a certain level or less.
Under these circumstances, a wide-band type front automatic power control (APC) disclosed in, for example, “High-Precision Laser Control Method (II) in Optical Disk Apparatus”, General Meeting in Spring 1991 of the Institute of Electronics, Information and Communications Engineering, C-372 by Taguchi and Hoshino, has been proposed as a means for suppress the laser noise while using a presently available semiconductor laser as a light source of the optical disk device.
In this method, when output light from the semiconductor is irradiated onto a recording medium, light actually irradiated onto an optical disk (that is, part of the front light of the semiconductor laser) is guided to a light detector, and with use of the detection signal, the output light of the semiconductor laser is controlled. Thus, the control band can be widened, and the optical laser noise can be suppressed.
In the wide band front APC method, it is significant how wide the control band can be made as compared to the reproduction signal band. The technique of widening the control band larger than the reproduction signal band is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 4-208581, U.S. Pat. No. 5,097,473 entitled “Semiconductor Laser Apparatus” and filed by the same applicants as those of the present invention.
This semiconductor laser apparatus is equipped with an error detector for generating an error signal corresponding to a difference between an output signal from a light intensity detector for detecting the intensity of output light from the semiconductor laser, and a laser drive control signal from outside, so as to constitute a feedback control system for negative-feedback-controlling the driving current for the semiconductor laser on the basis of the error signal. Further, the semiconductor laser apparatus includes a phase compensation circuit for negative-feed back a compensation current for compensating a phase delay of the feedback control system, to an input side of the error detector.
Further, the applicant of the present invention proposed “Semiconductor Laser Apparatus, Information Recording/Reproducing Apparatus and Image Recording Apparatus” capable of dispensing with the adjustment of the CR time constant for the phase compensation of a phase compensation loop separately from the front APC loop, and imparting the variable gain function to the phase compensation amplifier (Jpn. Pat. Appln. KOKAI Publication No. 8-83948, U.S. Pat. No. 5,579,329).
FIG. 1 shows the first prior art example of the semiconductor laser apparatus using a wide band front APC circuit, which is applied to an optical disk drive apparatus.
As shown in this figure, a semiconductor device for driving a laser, is denoted by reference numeral 60, and a semiconductor laser 1, a monitor light receiving element 2, a resistor element 3 and a variable resistor element 4 are provided outside the semiconductor device 50 to be connected thereto. In this example, the laser driving semiconductor device 60 has a plurality of external terminals. The semiconductor laser 1 is connected between the first external terminal 11 and the external power source, the monitor-use light receiving element 2 is connected between the second external terminal 12 and the external power, and the resistor element 3 is connected between the third external terminal 13 and the ground potential, and the variable resistor element 4 is connected between the fourth external terminal 14 and the ground potential.
The monitor-use light receiving element 2 is, for example, a pin photo-diode, having one end to which, for example, a power voltage Vcc is applied as a DC reverse voltage. The light receiving element 2 receives part of the light emission output from the semiconductor laser 1, and generates a current corresponding to the level of the received light. The light receiving element 2 and the semiconductor laser 1 are used in the optical recording/reproducing head portion of an optical disk drive apparatus.
Next, the interior of the semiconductor device 60 for driving the laser, will now be described.
A gain controllable operation amplifier circuit (gain control amplifier: GCA) 15 has a non-reversal input terminal (+), to which a reference voltage REF is input via an input resistance Ri.
To the reversal input terminal (−) of the circuit 15, a laser drive control signal SIG is input via an input resistance Ri. A negative feedback signal is input from the monitor-use light receiving element 2 to the circuit 15 via the second external terminal 12, and a gain control voltage is supplied from the variable resistor element 4 via the fourth external terminal 14, to the gain control terminal of the circuit.
A gain-fixed type control amplification circuit 16 is an operation amplification circuit having a non-reversal input terminal (+) and a reversal input terminal (−) to which output signals (differential signals) from the GCA 15 are input, and the signals are amplified.
The GCA 15 and the control amplifying circuit 16 connected to the next stage, constitute an error detection circuit for detecting an error between a laser drive control signal input and a negative feedback signal input from the monitor-use light receiving element.
A high-frequency signal generating circuit 17 is designed to generate a high frequency signal of a predetermined frequency, to be superimposed on a laser drive control signal SIG, so as to suppress laser noise caused by reflection light from the optical disk, returning to the semiconductor laser (that is, return light) during a play-back of the optical disk. The high-frequency signal is supplied to the non-reversal input terminal (+) of the GCA 15 via a coupling capacitor Cc.
A laser drive circuit 18 is designed to current-drive the semiconductor laser 1 on the basis of an output signal from the control amplifying circuit 16. The laser drive circuit 18 is made of, for example, an NPN transistor having a base connected to an output terminal of the control amplifying circuit 16, a collector connected to the first external terminal 11, and an emitter connected to the third external terminal 13.
In the feedback control system including the GCA 15, the control amplifying circuit 16, the laser drive circuit 18 and the laser optical system (that is, the semiconductor laser 1 and the monitor-se light receiving element 2), if it is necessary to correct the dispersion of the frequency bands of the GCA 15 and the control amplifying circuit 16, the resistance value of the variable resistor element 4 is adjusted to control the gain of the GCA 15. Thus, the loop gain of the feedback control system can be varied, and the loop band can be varied.
Further, the dispersion of the manner of the superimposition of the high frequency signal on the laser drive control signal SIG, can be corrected by adjusting the resistance value of the variable resistor element 4. It should be noted that the variable resistor element 4 is of a semi-fixed type, which is set in a fixed state after being adjusted by the maker which produces a product using the semiconductor laser apparatus, when it is shipped from the factory.
In the first prior art semiconductor laser apparatus, the feedback control system is constituted by the following structure, that is, the intensity of output light from the semiconductor laser 1 is detected by the monitor-use light receiving element 2, an error signal corresponding to the difference between the detected output signal and the laser drive control signal SIG is generated by the error detection circuit (the GCA 15 and the control amplifying circuit 16), and a drive current of the semiconductor laser 1 is negatively fed back on the basis of the error signal so that the voltage level of the reversal input terminal (−) of the GCA 15 becomes equal to the voltage level of the non-reversal input terminal (+).
FIG. 2A shows an example of the waveform of the laser drive control signal SIG.
The semiconductor laser is current-driven with multi-value control signal waveform, and thus an accurate light intensity modulation beam which is set proportional to the laser drive control signal, can be irradiated onto the optical disk. Consequently, it is possible to form an excellent record mark 11 on the optical disk as shown in FIG. 2B.
The frequency-to-gain characteristics of the GCA 15 and the control amplifying circuit 16 are expanded to about 200 to 300 MHz, and the frequency of the high-frequency signal is set at a high band region which is located slightly lower than the vicinity of the cutoff frequency.
However, if the gain of the GCA 15 is varied by adjusting the resistance value of the variable resistor element 4, for the purpose of correcting the dispersion of the characteristics of the laser optical elements, the level of the high-frequency signal superimposed onto the high band region of the laser drive control signal is varied, and therefore a sufficient laser noise suppression effect cannot be obtained.
FIG. 3 shows the second prior art semiconductor laser apparatus which uses a wide band front APC circuit, which is applied in, for example, a laser beam printer.
The second prior art example is similar to the first prior art example, except for the following points: (1) the high-frequency signal generating circuit 17, the GCA 15 and the variable resistor element 4 are omitted; (2) a level shift circuit 81 for shifting the level of the reference voltage is added, and an input resistor Ri is connected between the output node of the circuit and the non-reversal input terminal (+) of a control amplifying circuit 86; (3) a level shift circuit 82 for shifting the level of the laser drive control signal is added, and an input resistor Ri is connected between the output node of the circuit and the reversal input terminal (−) of the control amplifying circuit 86; and (4) a modulation circuit 83 in which a switch element 84 and a current source 85 are connected in series, intermittently controlled (modulated) by pulse signals, for controlling the switching of the irradiation of laser beam by the semiconductor laser 1 in the optical printing head portion, is added between the input node of the level shift circuit 82 and the ground potential node.
Each of the two level shift circuits 81 and 82 is made of emitter follower circuits each using an NPN transistor, which are connected in three steps.
In the second prior art semiconductor laser apparatus, as the switch element 84 is controlled intermittently and a laser drive control signal is input from the level shift circuit 82 to the reversal input terminal (−) of the control amplifying circuit 86.
However, in each of the level shift circuits 81 and 82, due to the dispersion of the voltage between the base and emitter of the transistor of the emitter follower circuits structured in three steps, the control accuracy is greatly deteriorated. Further, the base-emitter voltage varies along with time, and therefore the deterioration of the control accuracy is a very serious problem.
As described above, the conventional laser drive circuit entails the problem that if the gain of GCA is varied by adjusting the resistance value of the external variable resistor element, the level of the high-frequency signal superimposed onto the high band region of the laser drive control signal varies, and thus a sufficient laser noise suppression effect cannot be obtained.
Apart from the above, due to the dispersion of the voltage between the base and emitter of the transistor of each emitter follower circuit used in each level shift circuit for shifting the levels of the reference voltage and the laser drive control signal in the pre-stage of the error detection circuit, the control accuracy of the laser drive is significantly deteriorated.